The toggle-able free-spinning scroll wheel on the M705 was a game-changer for me. When I got the mouse, I was working for a place that had a lot of buggy spaghetti-code programs that interfaced with a massive database, and I often ended up manually scrolling through million-row tables in Toad correcting broken or missing data. It made my life sooo much easier.

I later upgraded to a MX Master ($100) which automatically enables free-spinning mode when you scroll quickly. It's fully adjustable, and it works super well.